Conventionally, an insulating system using a photocoupler is used for electrical insulation. This system, however, poses the problem that the current consumption is large and the system cannot be formed on the same semiconductor substrate as the peripheral circuit. A capacitor-type insulating system is described in JP-A-11-252270 as an insulating system which is smaller in current consumption and can be formed on the same semiconductor substrate as the peripheral circuit. The capacitor-type insulating system is configured of a filter including a capacitor and a resistor for cutting off the signal components in an area lower than a predetermined frequency. For signal transmission, what is called the differential transmission system is used, in which the change portion of the signal is transmitted.
In a system in which a plurality of field devices each having mounted thereon an interface device having the insulating function are connected to a network, each field device includes a power supply circuit supplied with a voltage (say, 24 V) from the network, which after voltage conversion, is supplied to the interface device itself. The power for the secondary circuit electrically insulated is supplied from another power supply on the secondary side. In this system, assume the power supply circuit of the interface device for supplying the power on the primary side runs out of order for some reason or other. The signal processing device on the secondary side, unable to know whether the circuit on the primary side is operating normally or not, continues to process the signal on the assumption that it is operating normally. Since the power supply circuit on the primary side of the network devices is out of order, on the other hand, correct information cannot be transmitted to the signal processing device on the secondary side, and in the worst case, the system is unavoidably stopped. In order to prevent this, the signal processing device on the secondary side is required to be informed whether the power supply circuit mounted on the field device on the primary side is operating correctly or not. In the conventional insulating system using the photocoupler, the output of the power supply circuit can be transmitted as it is to the secondary side through the photocoupler. In the capacitor-type insulating system, however, the output of the power supply circuit cannot be transmitted as it is as a state signal, since what is called the differential transmission system is used in which the change portion of the signal is transmitted. The state signal is defined as a signal which is maintained at high or low level and provides a significant signal in the absence of a change from high to low level or low to high level. Specifically, the change portion of the signal can be transmitted using the capacitor-type insulating system, but the state signal which remains unchanged cannot be transmitted using the capacitor-type insulating system. In an interface device using the capacitor-type insulating system, therefore, the state signal cannot be transmitted, so that the status of the power supply circuit cannot be transmitted to the secondary side. Neither the state signal such as a standby signal for controlling the primary circuit can be transmitted to the primary side from the signal processing device on the secondary side.
Secondly, the prior art consumes the power of the power supply circuit in a great amount, which poses a problem regardless of whether the devices are insulated or not. In the prior art, the series regulator has been used as a small voltage converter circuit. In the system having information processing devices connected to a network, however, the power equivalent to the number (N) of the devices connected is required to be supplied from the network. The power consumption, even though negligible for a single device, reaches an amount that cannot be ignored in a network connection. An increased power consumption increases the load on the network power supply, and the number of devices exceeding the power supply capacity cannot be connected. In other words, the problem is that the number (N) of the devices that can be connected to the network is limited.
Thirdly, in the case where the devices connected to the network are formed on a SOI (silicon on insulator) substrate, assume that the potential of the semiconductor substrate is floating. The surface-type devices operating at a voltage not higher than 5 V are not affected by the floating potential of the substrate, while the MOS transistor of high breakdown voltage type, in which current flows deep in the active layer, is easily affected by the substrate potential. In the case where the power supply circuit and the transceiver are incorporated into a single chip, the MOS transistor of surface operation type operating at a voltage not higher than 5 and the MOS transistor of high breakdown voltage are mounted together on the same semiconductor substrate. A floating substrate potential, therefore, poses the problem that the operation of the MOS transistor of high breakdown voltage becomes unstable.